This invention relates in general to transducers and in particular to a capacitance pressure transducer.
A capacitance pressure transducer provides a capacitance that varies in proportion to a sensed pressure. As shown in FIG. 1, a basic capacitance pressure transducer 1 includes two parallel plates 2 and 3 whose capacitance is determined by plate separation and dielectric material. A variation in pressure flexes the pressure plate 2 and decreases its separation from the fixed plate 3. The variation in capacitance can be used to change the response of a parallel circuit or to change the frequency of a resonant circuit. The response or frequency is proportional to the sensed pressure.
In certain applications, the capacitance pressure transducer must measure pressure while being subjected to high accelerations and vibrations. Because the pressure plate flexes in response to the accelerations and vibrations, the transducer falsely indicates a change in pressure.
To eliminate error due to vibration and acceleration, the fixed plate is replaced with at least one reference plate. The transducer 4 of FIG. 2 includes a pressure diaphragm 5 and two reference diaphragms 6 and 7. Electrodes on the pressure diaphragm 5 and one reference diaphragm 6 form a pressure capacitance, and electrodes on the reference diaphragms 6 and 7 form a reference capacitance. This transducer 4 is disclosed in U.S. Pat. No. 4,689,999 which is issued to Allied-Signal, Inc., the assignee of the present invention. When the transducer 4 is subjected to a steady-state acceleration or to a vibratory force, the diaphragms 5-7 move in phase. However, when the transducer 4 is subjected to a pressure, only the pressure diaphragm 5 moves. By analyzing the change in reference capacitance and subtracting it from the change in pressure capacitance, an accurate measurement of change in capacitance due solely to pressure is obtained. A ratio of the pressure and reference capacitances can be obtained from the oscillator disclosed in U.S. Pat. No. 4,987,782 which is also issued to Allied-Signal, Inc.
The diaphragms 5-7 and housing 8 of the transducer 4 are made of quartz and are fused together along the dotted lines 9. Resulting is a unitary structure.
However, the quartz has a relatively high thermal elastic coefficient, making it sensitive to temperature variations. The plot of FIG. 3 reveals the error as a percentage of full scale pressure for a temperature cycle between 225.degree. F. and -35.degree. F. ("down"/"up" refers to downside/upside of the temperature cycle). The variations in temperature give false indications of pressure changes because deflections are caused by temperature changes, not pressure changes.